Flares Sizing – Potential Hazardous Plume Considerations

When the potential exists for flare gas to contain toxic compounds (e.g., H2S, NH3), the possibility of loss of ignition of the flare stream requires consideration. In most such cases, an atmospheric dispersion analysis is performed for the potentially unburned relief stream flowing from the stack. Such an analysis should be performed for each relief contingency for which the relief gas is expected to contain a toxic compound. CRTC’s Health, Environment and Safety Group is normally consulted if dispersion modeling is required. The following guidelines apply to the atmospheric concentrations predicted by the modeling.

Toxic compound concentrations predicted by the modeling should not exceed the “Immediately Dangerous to Life and Health” (IDLH) level at any location accessible to personnel. The IDLH levels are specified by the National Institute for Occupational Safety and Health (NIOSH).

Toxic compound concentrations predicted by the modeling should not exceed levels set by local or federal regulatory agencies at any location to which the public has unrestricted access (typically outside the facility fence line).

In some cases, the potential may exist for the flare gas combustion products to contain toxic compounds (e.g., SO2). In these cases, dispersion analysis of the combustion product plume may be required to ensure that unacceptable concentrations of the hazardous products do not reach facility personnel or the public.